1. Field of the Invention
The present invention relates to a process for the recovery of magnesium oxide of high purity from contaminated magnesium containing starting materials or an industrial scale. First the starting substances are dissolved in hydrochloric acid and then pure magnesium oxide is formed from the magnesium chloride brine thus obtained by thermal decomposition after a pretreatment thereof in several steps. During the course of the process a purification and concentration takes place, where the impurities, especially iron compounds, are separated in form of oxide hydrates with simultaneous separation of silicic acid on the one hand and calcium in form of its sulfate being separated from the brine after addition of a material providing sulfate ions to the brine on the other hand. Optionally magnesium and/or calcium containing substances may be added to the brine to shift the pH-value into a range allowing or favoring the separation of the oxide hydrates, and the brine is brought into intimate contact with the waste gases of the decomposition furnace for the purpose of concentration.
2. Description of the Prior Art
In the known processes of this type the magnesium chloride brine or liquor respectivelly obtained by decomposition of the starting materials with hydrochloric acid is subjected to a first purification process, in which several impurities such as iron and aluminium are precipitated in the form of oxide hydrates and are subsequently separated, with silicic acid being separated too. Then calcium still being present in the magnesium chloride brine is reacted by addition of a material providing the solution with sulfate ions to give calcium sulfate, which is separated subsequently. Thereafter the brine thus purified is brought into intimate contact with the waste gases of the decomposition furnace so as to effect a better heat balance of the process. This contact is effected in a device constructed generally in the form of a wet washer. In addition to combustion gases, the waste gases of the furnace contain hydrogen chloride and water vapor resulting from the decomposition of the magnesium chloride brine. These waste gases frequently also contain some solid material in form of dust which has been delivered from the furnace by the strong smoke gas stream prevailing in the furnace.
The performance of this known process requires a relatively large apparatus and working expenditures, as in each of the two precipitating and separating processes to be carried out subsequently the pH must be adjusted to a value favorable for the precipitation and separation of the impurities. In addition to the substantial expenditure of work and the vessels necessary therefor, a further disadvantage is that the substances added for adjusting the pH-value for the precipitation of calcium remain in the brine and need to be therefore uncontaminated, since otherwise they would impair the desired purity of the magnesium oxide to be obtained. Analogous considerations in particular apply to the necessary adjustment of the pH-value of the magnesium chloride brine purified already after its emergence from the concentrator, before it is subjected to thermal decomposition. Therefore, only pure magnesium oxide, which means only the end product of the process, may be added for increasing the pH-value of the brine, which is acid as a result of taking up hydrogen chloride in the concentrator, so that the economics are impaired.